1. Field of the Invention
This invention relates to crossed-field plasma switches, and to cold cathodes used therein.
2. Description of the Related Art
A low pressure plasma opening switch, referred to as the CROSSATRON Modulator Switch (CROSSATRON is a trademark of Hughes Aircraft Company, the assignee of the present invention), has recently been developed. Details of this switch are provided in U.S. Pat. No. 4,596,945 by Schumacher, et al., assigned to Hughes Aircraft Company, and in a text edited by Guenther, et al., Opening Switches, chapter entitled "Low-Pressure Plasma Opening Switches", Schumacher, et al., pages 93-129, Plenum Publishing Corp., 1987. The switch is a secondary-electron-emitter, cold cathode device which employs a controlled diffuse discharge to both close and open pulsed-power circuits at high speed and high repetition frequency. In contrast to prior DC-current opening-switch devices such as hard-vacuum tetrodes, the low-pressure plasma opening switch described by Schumacher eliminates the need for a cathode heater, and offers instant starting, long life, low forward voltage drop, high current conduction and electromechanically rugged operation.
The basic configuration of the switch is illustrated in FIG. 1. The switch is based upon a crossed-field discharge in a four element, coaxial system consisting of a cold cathode 2, an anode 4, and a source grid 6 and control grid 8 between the cold cathode 2 and anode 4. These elements are cylindrical in shape; FIG. 1 depicts a sectional view on one side of the device center line.
Charges for conduction are generated by a plasma discharge near the cathode. The plasma is produced by a crossed-field cold cathode discharge in a gap located between the source grid 6 (which serves as an anode for the local cross-field discharge) and the cathode 2. The gap is magnetized with a cusped field supplied by permanent magnets 10 attached to the outside of the switch. This arrangement eliminates the need for cathode heater power, and also permits instant start operation.
The source plasma 12 is generated by pulsing the potential of the source grid 6 to a level above 500 volts for a few microseconds to establish a crossed-field discharge. When equilibrium is reached, the source grid potential drops to the low discharge level about 500 volts above the potential of cold cathode 2. With the control grid 8 remaining at the cathode potential, the switch remains open and the full anode voltage appears across the vacuum gap between the control grid 8 and the anode 4.
The switch is closed by releasing the control grid 8 potential, or by pulsing it momentarily above the 500 volt plasma potential. This allows plasma to flow through the source grid 6 and control grid 8 to the anode 4. Electrons from the plasma are collected by the anode, the switch conducts, and the anode voltage falls to the 500 volt level. To open the device, the control grid 8 is returned to the cathode potential or below in a hard tube fashion.
Once a glow discharge has been initiated, it is maintained as illustrated in FIG. 2 by secondary electron emission from the cold cathode. This is illustrated in FIG. 2, which plots the steady state, glow-discharge potential distribution between the cathode and anode. The plasma potential relative to the cathode is generally 200-1,000 volts, depending upon the gas species and electrode materials used, as well as the current density at the cathode. Ions are collected from the plasma in the gap across non-neutral sheath regions 14, 16 at both the cathode and anode, respectively. Electrons, however, are collected at the anode only. The plasma maintains a small anode-sheath voltage drop to adjust the ambipolar flux of electrons and ions so that the plasma remains electrically neutral. Most of the potential drop across the switch occurs at the cathode sheath 14, where ions are accelerated to kinetic energy levels sufficient to stimulate the emission of secondary electrons from the cathode surface. The total cathode current is thus the sum of the ion current collected from the plasma (current flow 18), and the emitted secondary-electron current from the cathode (current flow 20). Electrons from the plasma are repelled by the cathode potential, and cannot cross the cathode sheath 14 to reach the cathode (current path 22).
Following their emission from the cathode, the secondary electrons are accelerated through the cathode sheath 14 and enter the plasma at an energy corresponding to the 200-1,000 volt cathode sheath drop. The magnetic field traps these electrons in a spiral between the cathode and anode, causing them to undergo ionizing collisions with the background neutral gas atoms in the plasma before they are collected by the anode. In the steady state, the rate of ionization from these collisions balances the ion loss rate to the cathode and anode such that the glow-discharge plasma is maintained at a constant level.
The cold cathode has typically been formed from a high strength, relatively inexpensive stainless steel or copper tube, with a smooth-bore refractory metal sheet, typically molybdenum, vacuum oven brazed to the inside surface of the tube to provide an electron-emissive surface facing the plasma. This process is expensive because the large area braze requires a significant amount of gold-based braze material, vacuum oven time, and tooling. Process yield has also not been satisfactory because of differences in the thermal expansion properties of the refractory metal sheet and underlying tube material. For example, molybdenum and copper have different rates of thermal expansion. The molybdenum sheet is brazed to the tube at a temperature of about 950.degree. C., but when the sheet cools, it contracts less than the underlying copper tube. This process produces wrinkles in the molybdenum sheet, a poor bond, and trapped pockets of air and gold braze.
The efficiencies achieved with such switches have also not been optimum. The efficiency is directly proportional to the forward voltage drop across the switch. The forward drop could theoretically be reduced by increasing the secondary electron yield from the cold cathode and/or increasing the dwell time of the secondary electrons within the plasma, thereby increasing the probability of an electron colliding with and ionizing a gas molecule before being captured by the anode. With a plasma potential of 500 volts, current switches achieve a secondary electron yield of only about 0.2 per ion striking the cathode wall. While the secondary electron yield could in principle be increased by coating the cathode with a very low work function material, such materials are normally sputtered away by the plasma ions which strike the cathode. Although molybdenum is most frequently used as a cathode coating, it is expensive and difficult to work with.